Document CN102275154 discloses a hydraulic spreader comprising separating arms actuated by a hydraulic piston. This type of spreader can be used in bodywork, in particular to straighten the longerons of the chassis of a vehicle that are curved due to an accident. Once the longerons are straightened using a straightening bracket, a crease is typically observed at the initial curve. It is then necessary to insert a spacing tool inside the longeron, which is generally a beam with a hollow section. It has, however, been able to be observed that the device disclosed in the prior art document was difficult to use and did not offer optimal performance to cause the creases observed in the longerons to disappear.